Vertical cylindrical skein of hollow fiber membranes and method of maintaining clean fiber surfaces

ABSTRACT

A gas-scrubbed vertical cylindrical skein of “fibers” has their opposed terminal portions held in headers unconfined in a modular shell, and aerated with a cleansing gas supplied by a gas-distribution means which produces a mass of bubbles serving the function of a scrub-brush for the outer surfaces of the fibers. The skein is surprisingly effective with relatively little cleansing gas, the specific flux through the membranes reaching an essentially constant relatively high value because the vertical deployment of fibers allows bubbles to rise upwards along the outer surfaces of the fibers. The effectiveness is critically dependent upon the length of each fiber in the skein. That length is in the range from at least 0.1% more than the fixed distance between opposed faces of the skein&#39;s headers, but less than 5% greater than the fixed distance. Lack of tension allows the fibers to sway in bubbles flowing along their outer surfaces making them surprisingly resistant to being fouled by buildup of deposits of inanimate particles or microorganisms in the substrate. For use in a large reservoir, a bank of skeins is used with a gas distributor means which has fibers preferably &gt;0.5 meter long, which together provide a surface area &gt;10 m 2 . The terminal end portions of fibers in each header are kept free from fiber-to-fiber contact with a novel method of forming a header.

BACKGROUND OF THE INVENTION

This is a continuation-in-part application of Ser. No. 08/514,119 filedAug. 11, 1995 U.S. Pat. No. 5,639,373. Terms used in the parent case aresummarized in a glossary herein to shorten the specification; and, toavoid repetition herein, additional details in the parent case as wellas in provisional application Ser. No. 60/012,921 filed Mar. 6, 1996,are incorporated herein by reference thereto as if fully set forthherein. In particular, considerations relative to the prior art anddetails of operation of prior art devices, all of which have been setforth in the '119 parent and provisional applications, are incorporatedherein by reference thereto as if fully set forth herein.

This invention relates to a membrane device which is an improvement on aframeless array of hollow fiber membranes and a method of maintainingclean fiber surfaces while filtering a substrate to withdraw a permeate,which is also the subject of U.S. Pat. No. 5,248,424; and, to a methodof forming a header for a skein of fibers.

This invention is particularly directed to relatively large systems forthe microfiltration of liquids, and capitalizes on the simplicity andeffectiveness of a configuration which dispenses with forming a modulein which the fibers are confined. As in the '424 patent, the novelconfiguration efficiently uses air discharged near the base of a skeinto produce bubbles in a specified size range, and in an amount largeenough to scrub the fibers, and to provide controlled scrubbing offibers one against another (“inter-fiber scrubbing”). Unlike in the '424system the fibers in a skein are vertical and do not present an arcuateconfiguration above a horizontal plane through the horizontalcenter-line of a header. As a result, the path of the rising bubbles isgenerally parallel to the fibers and is not crossed by the fibers of avertical skein. Yet the bubbles scrub the fibers.

The restrictedly swayable fibers, because of their defined length, donot get entangled, and do not abrade each other excessively, as islikely in the '424 array.

The side-to-side displacement of an intermediate portion of each fiberwithin the “zone of confinement” or “bubble zone” is restricted by thefiber's length. The defined length of the fibers herein minimizes (i)shearing forces where the upper fibers are held in the upper header,(ii) excessive rotation of the upper portion of the fibers, as well as(ii) excessive abrasion between fibers. Such swaying motion of a fiberwith side-to-side displacement is distinct from vibration which occurswhen a fiber is taut, that is, when the length of the ported fiberexposed to substrate is not longer than the distance between the opposedfaces of upper and lower headers holding the fiber. Such vibration isinduced by bubbles in a process for exfoliating and precipitating denseparticles in U.S. Pat. No. 5,209,852 to Sunaoka et al. Unlike the fibersheld in the module used in the '852 process, in our novel skein, thereis essentially no tension on each fiber because the opposed faces of theheaders are spaced apart at a distance less than the length of anindividual fiber.

The use of an array of fibers in the direct treatment of activatedsludge in a bioreactor, is described in an article titled “DirectSolid-Liquid Separation Using Hollow Fiber Membrane in an ActivatedSludge Aeration Tank” by Kazuo Yamamoto et al in Wat. Sci. Tech. Vol.21, Brighton pp 43-54, 1989, and discussed in the '424 patent thedisclosure of which is incorporated by reference thereto as if fully setforth herein. The relatively poor performance obtained by Yamamoto et alwas mainly due to the fact that they did not realize the criticalimportance of maintaining flux by aerating a skein of fibers from withinand beneath the skein. They did not realize the necessity of thoroughlyscrubbing substantially the entire surfaces of the fibers by flowingbubbles through the skein to keep the fibers awash in bubbles. Thisrequirement becomes more pronounced as the number of fibers in the skeinincreases.

Tests using the device of Yamamoto et al indicate that when the air isprovided outside the skein the flux decreases much faster over a periodof as little as 50 hr, confirming the results obtained by them. This isevident in FIG. 1 described in greater detail below, in which the graphsshow results obtained by Yamamoto et al, and the '424 array, as well asthose with a vertical skein in which the headers are rectangular, allthree assemblies using essentially identical fibers, under essentiallyidentical conditions.

The investigation of Yamamoto et al with downwardly suspended fibers wascontinued and recent developments were reported in an article titled“Organic Stabilization and Nitrogen Removal in Membrane SeparationBioreactor for Domestic Wastewater Treatment” by C. Chiemchaisri et aldelivered in a talk to the Conference on Membrane Technology inWastewater Management in Cape Town, South Africa, Mar. 2-5, 1992, alsodiscussed in the '424 patent. The fibers were suspended downwardly andhighly turbulent flow of water in alternate directions, was essential.

It is evident that the disclosure in either the Yamamoto et al or theChiemchaisri et al reference indicated that the flow of air across thesurfaces of the suspended fibers did little or nothing to inhibit theattachment of microorganisms from the substrate.

Later, in European patent application 0 598 909 A1 filed by Yamamori etal, they sought to avoid the problem of build-up on the fibers by“spreading the hollow fibers in the form of a flat sheet” (see page 4,lines 46-7) and there is no indication how the fibers would bemaintained in a spread position in actual use. Further, each array isheld in a “structural member for enclosing and supporting the fasteningmember” (see page 3, line 42, and lines 51-52) which is contrary to theconcept of a frameless array. Their FIGS. 14, and 18 emphasize thehorizontal configuration in which the array is used. To combat build-upFIG. 13 depicts how the fibers would trough when the array is taken outof the reservoir to be “vibrated” or shaken. A prior art module isillustrated in FIG. 16 showing both ends of each fiber potted in acylindrical header, each fiber forming a loop, the looped ends beingfree. As the data in FIG. 17 shows, use of the prior art cylindricalmodule with looped ends freely movable in the substrate, was lesseffective than the frameless array with spread apart looped fibers shownin FIG. 1.

SUMMARY OF THE INVENTION

It has been discovered that for no known reason, fibers which are morethan 5% but less than 10% longer than the fixed distance between theopposed faces of the headers of a vertical skein, tend to shear off atthe face; and those 10% longer tend to dump up in the bubble zone; and,that a gas-scrubbed vertical cylindrical skein of substantiallyconcentrically disposed, restrictedly swayable fibers, provides anoptimum configuration of fibers through which bubbles of afiber-cleansing gas (“scrubbing gas”) when flowed vertically upwards,parallel to and along the surfaces of the fibers. In a skein of denselypacked fibers, bubbles in such a configuration are more effectivecleansing agents than bubbles which are intercepted by arcuate fiberscrossing the path of the rising bubbles. Bubbles of an oxygen-containinggas to promote growth of microbes unexpectedly fails to build-up growthof microbes on the surfaces of swaying fibers because the surfaces are“vertically air-scrubbed”. Deposits of animate and/or inanimateparticles upon the surfaces of fibers are minimized when therestrictedly swayable fibers are kept awash in codirectionally risingbubbles which rise with sufficient velocity to exert a physicalscrubbing force (momentum provides the energy) to keep the fiberssubstantially free of deleterious deposits. Thus, an unexpectedly highflux is maintained in fibers over each unit area the surface of theskein fibers over a long period.

In a “gas-scrubbed assembly” comprising a skein and a gas-distributionmeans, the skein preferably has a surface area which is at least >1 m²,and opposed spaced-apart ends of the fibers are secured in spaced-apartheaders, so that the fibers, when deployed in the substrate, acquire agenerally vertical cylindrical profile within the substrate and swayindependently within the bubble zone defined by at least one column ofbubbles. The length of fibers between opposed surfaces of headers fromwhich they extend, is in a critical range from at least 0.1% (percent)longer than the distance separating those opposed faces, but less than5% longer. Usually the length of fibers is less than 2% longer, and mosttypically, less than 1% longer, so that sway of the fibers is confinedwithin a vertical zone of movement, the periphery of which zone isdefined by side-to-side movement of outer skein fibers; and, themajority of these fibers move in a slightly larger zone than one definedby the projected area of one header upon the other. Though the distancebetween headers is fixed during operation, the distance is preferablyadjustable to provide an optimum length of fibers, within the aforesaidranges, between the headers.

Permeate may be withdrawn from only one, usually the upper permeatecollection means (pan or end-cap), or, in skeins of large surface areagreater than 200 m², from both (upper and lower) pans or end-caps. Mostpreferably, air is introduced between skein fibers by an air-tube pottedcentrally axially within the upper end-cap, the air-tube supplying airto a sparger near the base of the skein fibers, and simultaneouslyproviding a spacer means to position and space the lower end-cap therequisite distance from the upper end-cap. The sparger is part of agas-supply means which supplies cleansing gas. The air-tube may beinternally provided with a concentric permeate withdrawal tube axiallyextending to the permeate collection zone in the lower end-cap, and inopen fluid communication with it to withdraw permeate from both theupper and lower end-caps. Alternatively, the permeate withdrawal tubefrom the lower end-cap may be externally disposed so as to withdrawpermeate from a passage in the lower portion of the end-cap, the tubebeing led outside the skein fibers, to communicate with the permeatewithdrawal tube from the upper end-cap.

Preferably, for maximum utilization of space on a header, the fibers aredeliberately set in a spiral pattern by rolling a large array into aspiral roll and potting each end of the spiral roll directly in acylindrical resin-confining means. Such resin-confining means istypically a cylindrical end-cap such as is used for PVC pipe, or, anopen-ended cylindrical ring. For use, each ring of the skein is, inturn, secured in an end-cap. Whether directly potted in an end-cap, orfirst in a ring, men secured in an end-cap, an integral header isformed. Since, a cylindrical skein in use, requires an end-cap to serveas an integral header, an end-cap integral header will be referred tohereafter as an “end-cap” for brevity.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and additional objects and advantages of the inventionwill best be understood by reference to the following detaileddescription, accompanied by schematic illustrations of preferredembodiments of the invention, in which illustrations like referencenumerals refer to like elements, and in which:

FIG. 1 is a graph in which the variation of flux is plotted as afunction of time, showing three curves for three runs made with threedifferent arrays, in each case, using the same amount of air, theidentical membranes and the same membrane surface area. The resultsobtained by Yamamoto et al are plotted as curve 2 (under conditionsmodified to give them the benefit of doubt as to the experimentalprocedure employed, as explained below); the flux obtained using thegas-scrubbed assembly of the '424 patent is shown as curve 1; and theflux obtained using a gas-scrubbed assembly of equal surface area isshown as curve 3. The headers of the gas-scrubbed assembly arerectangular parallelpipeds.

FIG. 2 is a cross-sectional view schematically illustrating acylindrical skein having upper and lower end-cap integral headers ineach of which is directly potted an array of fibers in a finished headersealed at its periphery to the wall of the end-cap without a gasket;permeate is withdrawn separately from the upper and lower headers andthe draw from each combined in a permeate withdrawal manifold. Exceptfor the lower end-cap resting on the floor of the tank, or otherwisesupported in the substrate, the skein is unsupported during operation.By “unsupported” is meant ‘not supported except for spacer means tospace the headers’.

FIG. 2A is a bottom plan view of a potted array held as a roll in afiber-setting form, before the end of the roll is potted in a ring, soas to form an integral header in which the pattern of fibers is spiral.

FIG. 2B is a bottom plan view of a series of potted cylindrical arraysreferred to as “ring arrays” because the ends are secured in stiffcylindrical rings, the arrays being nested with each successive ringarray being slid over the previous one. The nested rings are then pottedin a resin-confining ring.

FIG. 2C is a bottom plan view of a series of planar arrays, the widthsof each being chosen so that they may be stacked, chord-like (that is,as successive chords in the resin-confining ring) before the stack ispotted in the ring.

FIGS. 3 and 3A are a cross-sectional view schematically illustrating acylindrical skein and end-cap integral headers as in FIG. 2, except thatpermeate is withdrawn from only the upper header.

FIG. 4 is a perspective view of a single array, schematicallyillustrated, of a row of substantially coplanarly disposed parallelfibers secured near their opposed terminal ends between spaced apartcards. Typically, a single such array with a large number of fibers isrolled up before being sequentially potted.

FIG. 5 illustrates a side elevational view of a cylindrical skein offibers, with the ends of the fibers on a rolled strip potted in a ringclamped to a panel coated with a release coating, to describe in detailhow a finished header is formed.

FIG. 6 is a side elevational view schematically illustrating anotherembodiment of a cylindrical skein in which a conventionally formedheader is hend in a permeate pan and permeate is withdrawn from thelower end-cap into the upper end-cap through a rigid permeate tubeinserted through both the upper and lower headers. Terminal portions ofa permeate connector tube are held in fluid-tight engagement with theupper and lower headers so that the permeate tube functions as a spacermeans, and at the same time, as a support for the upper end-cap.

FIG. 7 is a side elevational view schematically illustrating acylindrical skein in which a ring header is formed first. The ringheader is then sealed into an end-cap. In addition to the permeate tube,a rigid air supply tube is inserted through the upper end-cap and upperheader into the central portion of the skein, the lower portion of theair supply tube being potted in the lower header, thus functioning as aspacer means, and at the same time, as a support for the upper end-cap.

FIG. 8 illustratively shows another embodiment of the skein in which thepermeate tube is concentrically disposed within the air supply tube, andboth are potted, near their lower ends in the lower header. Ports in thelower end of the air supply tube provide air near the base of the skeinfibers.

FIG. 9 is a perspective view schematically illustrating a pair of skeinsin a bank in which the upper headers are supported on brackets on thevertical wall of a tank and the lower headers rest on the floor. Theskeins in combination with a gas-distribution means form a“gas-scrubbing assembly” deployed within a substrate, with the fiberssuspended essentially vertically in the substrate.

FIG. 10 is an elevational view schematically illustrating a“stand-alone” cluster of skeins.

FIG. 11 is an elevational view schematically illustrating a bank ofskeins mounted against the wall of a bioreactor, showing the convenienceof having all piping connections outside the liquid.

FIG. 12 is a plan view of the bioreactor shown in FIG. 11 showing howmultiple banks of skeins may be positioned around the circumference ofthe bioreactor to form a large permeate extraction zone while aclarification zone is formed in the central portion with the help ofbaffles.

FIG. 13 is a bar graph showing flux (liters per meter² per hour, LMH) asa function of the orientation of a skein.

FIG. 14 is a graph in which flux is plotted as a function of time duringwhich a vertical cylindrical skein is aerated at a constant flow rate ofair provided in one instance, by external aeration, and in anotherinstance, by internal aeration.

FIG. 15 is a graph in which flux is plotted as a function of time forthe same cylindrical skein used in two different embodiments achieved byadjusting the distance between the headers; the first embodiment havingspaced apart conventionally at the maximum distance to provide tautfibers, and the outer having headers spaced closer together to provideswayable fibers. During the test each vertical cylindrical skein isaerated at a constant flow rate of air.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The Cylindrical Skein and the Arrays which form it

The cylindrical skein of this invention may be used in a liquid-liquidseparation process of choice, and more generally, in various separationprocesses. The skein is specifically adapted for use in microfiltrationprocesses used to remove large organic molecules, emulsified organicliquids and colloidal or suspended solids, usually from water. Typicalapplications are (i) in a membrane bioreactor, to produce permeate aspurified water and recycle biomass; for (ii) filtration of secondaryeffluent from wastewater treatment, to remove suspended solids andpathogenic bacteria; (iii) clarification of aqueous streams includingfiltration of surface water to produce drinking water (removal ofcolloids, long chain carboxylic acids and pathogens); (iv) separation ofa permeable liquid component in biotechnology broths; (v) dewatering ofmetal hydroxide sludges; and, (vi) filtration of oily wastewater, interalia.

Typically the skein is configured so that all connections for fluidsentering or leaving the skein are provided in the upper header. Permeateis most preferably withdrawn through a tube passing through the upperheader whether (i) the lower header collects no permeate (as explainedbelow); or (ii) permeate collects in both the upper and lower headers.The substantially circumferential geometry of the potted skein fibers isdetermined by a ‘fiber-setting form’ used to set the fibers in a ringbefore they are potted. Instead of a single array rolled into a spiral,the fibers may be arranged in plural concentric arrays, or in pluralflat arrays arranged chord-like in the potting ring. After potting, astar-shaped sparger or other shaped gas-distribution means is positionednear the base of the skein fibers. The skein preferably operates in asubstrate held in a reservoir at atmospheric pressure or above in therange up to about 10 atm in a pressurized vessel, without being confinedwithin the shell of a module.

One or more arrays are substantially concentrically potted about acentral vertical axis in headers, the surfaces of which are inhorizontal (x-y) planes. Instead of a single continuous array, pluralarrays may be made and joined together, end-to-end successively, to forma much larger array which can be extended as it is rolled into a spiralroll.

Operation of the cylindrical skein is affected by (a) the fiber chosen,(b) the amount of air used, and (c) the substrate to be filtered. Thegoal is to filter a slow-moving or captive substrate in a largecontainer under ambient or elevated pressure, but preferably underessentially ambient pressure, and to maximize the efficiency of a skeinwhich does so (filters) practically and economically.

By operating at ambient pressure, mounting the integral headers of theskein within a reservoir of substrate, and by allowing the fibersrestricted movement within the bubble zone in a substrate, we minimizedamage to the fibers. Because, a header secures at least 10, preferablyfrom 50 to 50,000 fibers, each generally at least 0.5 m long, in askein, it provides a high surface area for filtration of the substrate.

The Fibers and How they are Densely Packed

The fibers divide a reservoir into a “feed zone” and a withdrawal zonereferred to as a “permeate zone”. The feed of substrate is introducedexternally (referred to as “outside-in” flow) of the fibers, andresolved into “permeate” and “concentrate” streams. The skein, or a bankof skeins of this invention is most preferably used for microfiltrationwith “outside-in” flow. Though at least one skein is replaceablydisposed in a small reservoir having a volume up to about 10 L (liters)and even up to about 100 L or more, a bank of skeins is preferably usedin a relatively large reservoir having a volume in excess of 1000 L,such as a flowing stream, more typically a pond or tank. Most typically,a bank or plural banks with collection means for the permeate, aremounted in a large tank under atmospheric pressure, and permeate iswithdrawn from the tank.

Where a bank or plural banks of skeins are placed within a tank orbio-reactor, and no liquid other than the permeate leaves the tank, itis referred to as a “dead end tank”. Alternatively, a bank or pluralbanks may be placed within a bioreactor, permeate removed, and sludgedisposed of; or, in a tank or clarifier used in conjunction with abioreactor, permeate removed, and sludge disposed of.

The fibers used to form the skein may be formed of any conventionalmembrane material provided the fibers are flexible and have an averagepore cross sectional diameter for microfiltration, namely in the rangefrom about 1000 Å to 10000 Å. Typically fibers range from 1 m to about 5m long, depending upon the dimensions of the body of substrate (depthand width) in which the skein is deployed. Preferred fibers operate witha transmembrane pressure differential in the range from 7 kPa (1 psi)-69kPa (10 psi) and are used under ambient pressure with the permeatewithdrawn under gravity. The fibers are chosen with a view to performtheir desired function, and the dimensions of the skein are determinedby the geometry of the headers and length of the fibers. It isunnecessary to confine a skein in a modular shell, and a skein is not.

For hollow fiber membranes, the outside diameter of a fiber is at least20 μm and may be as large as about 3 mm, typically being in the rangefrom about 0.1 mm to 2 mm. The larger the outside diameter the lessdesirable the ratio of surface area per unit volume of fiber. The wallthickness of a fiber is at least 5 μm and may be as much as 1.2 mm,typically being in the range from about 15% to about 60% of the outsidediameter of the fiber, most preferably from 0.5 mm to 1.2 mm.

The number of fibers in a single array is arbitrary, typically being inthe range from about 1000 to about 10000 for commercial applications,and the preferred surface area for a skein is in the range from 10 m² to100 m². The center to center distance of adjacent fibers is preferablyin the range from 1.2 (1.2d) to about 5 times (5d) the outside diameter‘d’ of a fiber. Preferred center-to-center spacing is from about 1.5d to2d. The packing density of fibers, that is, the number of fibers perunit area of header preferably ranges from 4 to 50 fibers/cm² dependingupon the diameters of the fibers.

The particular method of securing the fibers in each of the headers isnot narrowly critical, the choice depending upon the materials of theheader and the fiber, and the cost of using a method other than potting.However, it is essential that each of the fibers be secured influid-tight relationship within each header to avoid contamination ofpermeate. This is effected by potting the fibers essentially vertically,in closely-spaced relationship, substantially concentrically.

Preferred fibers are made of organic polymers and ceramics, whetherisotropic, or anisotropic, with a thin layer or “skin” on the outsidesurface of the fibers. Fibers may be made from braided yarn covered witha water-insoluble polymeric material such as those disclosed in U.S.Pat. No. 5,472,607. Preferred organic polymers for fibers arepolysulfones, poly(styrenes), including styrene-containing copolymerssuch as acrylonitrile-styrene, butadiene-styrene andstyrene-vinylbenzyl-halide copolymers, polycarbonates, cellulosicpolymers, polypropylene, polyvinyl chloride), poly(ethyleneterephthalate), and the like disclosed in U.S. Pat. No. 4,230,463 thedisclosure of which is incorporated by reference thereto as if fully setforth herein. Preferred ceramic fibers are made from alumina, by E. I.duPont deNemours Co. and disclosed in U.S. Pat. No. 4,069,157.

The Headers

One integral header of a skein is displaceable in any direction relativeto the other, either longitudinally (x-axis) or transversely (y-axis),only prior to submerging the skein for operation. To use a skein, theheaders are vertically spaced apart in parallel relationship within areservoir, for example, by mounting one header above another against avertical wall of the reservoir which functions as a spacer means. Thisis also true prior to spacing one header directly above another withother spacer means such as bars, rods, struts, I-beams, channels, andthe like, to assemble plural skeins into a “bank or cluster of skeins”(“bank” for brevity). After assembly into a bank, a segment intermediatethe potted ends of each individual fiber is displaceable along eitherthe x- or the y-axis, because the fibers are loosely held in the skein.

Because each integral header is preferably directly potted in a ring ofsuitable material from which the header of cured potting resin is notremoved, no gasket is required (hence referred to as “gasketless”)between the cured resin of the header and the inner periphery of thering. When the integral header is adhesively secured in an end-cap toform a permeate-collection zone, again, no gasket is required, thoughone may be used if the integral header is to be disassembled.

The fixing material to fix the fibers in a finished header (or fixinglamina) is most preferably either a thermosetting or thermoplasticsynthetic resinous material, optionally reinforced with glass fibers,boron or graphite fibers and the like. Thermoplastic materials may becrystalline, such as polyolefins, polyamides (nylon), polycarbonates andthe like, semi-crystalline such as polyetherether ketone (PEEK), orsubstantially amorphous, such as poly(vinyl chloride) (PVC),polyurethane and the like. Thermosetting resins commonly includepolyesters, polyacetals, polyethers, cast acrylates, thermosettingpolyurethanes and epoxy resins. Most preferred as a “fixing” material(so termed because it fixes the locations of the fibers relative to eachother) is one which when cured is substantially rigid in a thickness ofabout 2 cm, and referred to genetically as a “plastic” because of itshardness. Such a plastic has a hardness in the range from about Shore D30 to Rockwell R 110 and is selected from the group consisting of epoxyresins, phenolics, acrylics, polycarbonate, nylon, polystyrene,polypropylene and ultra-high molecular weight polyethylene (UHMW PE).Polyurethane such as is commercially available under the brand namesAdiprene® from Uniroyal Chemical Company and Airthane® from AirProducts, and commercially available epoxy resins such as Epon 828 areexcellent fixing materials.

The resulting membrane device comprises, (i) a vertical cylindricalskein of a multiplicity of restrictedly swayable fibers, together havinga surface area in the range from 1 m² to 1000 m², preferably from 10 m²to 100 m², secured only in spaced-apart headers; and (ii) agas-scrubbing means which produces a column of bubbles rising within andnear the base of the skein, and engulfing the skein. Bubbles generatedhave an average diameter in the range from about 0.1 mm to about 25 mm,or even larger. A fluid component is selectively removed from thesubstrate.

The Gas-Scrubbed Assembly

A gas-scrubbed assembly comprises, (a) at least one skein, or a bank ofgas-scrubbed cylindrical skeins of fibers which separate a desiredpermeate from a large body of multicomponent substrate having finelydivided particulate matter in the range from 0.1 μm-44 μm dispersedtherein, (b) each skein comprising at least 20 fibers having upper andlower terminal portions potted spaced-apart, in upper and lower end-capintegral headers (“end-caps”), respectively, the fibers beingrestrictedly swayable in a bubble zone, and (c) a shapedgas-distribution means adapted to provide a profusion of verticallyascending bubbles in a column above and in close proximity to the upperface of the lower header, the length of the fibers being from at least0.1% but less than 5% greater than the distance between the opposedfaces of the headers. The shaped gas-distribution means hasthrough-passages therein through which gas is flowed, continuously orintermittently, at a flow rate which is proportional to the number offibers. The flow rate is generally in the range from 0.47-14 cm³/sec perfiber (0.001-0.03 scfm/fiber) (standard ft³ per minute per fiber),typically in the range from 1.4-4.2 cm³/sec/fiber (0.003-0.009scfm/fiber). The surface area of the fibers is not used to define theamount of air used because the air travels substantially verticallyalong the length of each fiber.

The gas-scrubbed assembly is used (i) in combination with verticallyadjustable spacer means for mounting the headers in vertically spacedapart relationship, in open fluid communication with (ii) collectionmeans for collecting the permeate; means for withdrawing the permeate;and, (iii) sufficient air to generate enough bubbles flowing upwardlythrough the skein, between and parallel to the fibers so as to keep thesurfaces of the fibers substantially free from deposits of livemicroorganisms as well as small inanimate particles which may be presentin the substrate.

With surprisingly little cleansing gas discharged from a spargerdisposed between fibers near their base, the specific flux atequilibrium is maintained over a long period, typically from 50 hr to1500 hr. The sparger of a gas-distribution means is disposed adjacentthe upper (“fore”) face of the lower header to generate a column ofrising bubbles within which column the fibers are awash in bubbles. Abank of skeins may additionally be “gas-scrubbed” with one or moreair-tubes disposed between the lower headers of adjacent skeins, mostpreferably, also adjacent the outermost fibers of the first and lastskeins, so that for “n” headers there are “n+1” additional air-tubes.Each end-cap is preferably a commercially available synthetic resinous“dish” typically provided for the shell of a heat exchanger, or a “cap”for a pipe having a diameter about the same as the cylindrical skein tobe formed. The upper and lower headers are cylindrical discs having thesame diameter, and plural such skeins may be clustered in a single row,or multiple rows, or in a honeycomb cluster, the upper headers beinginterconnected for support, and the lower headers supported on the floorof the reservoir. Even skeins of different diameters may be clustered asdescribed, if the headers are adequately intersupported in thesubstrate. Appropriately positioned and interconnected gas-tubes extendfrom a gas (air) manifold to service the bank, and an appropriatemanifold is provided to withdraw permeate. The type of gas (air)manifold is not narrowly critical provided it delivers bubbles in apreferred size range from about 0.1 mm to 25 mm, measured within adistance of from 1 cm to 50 cm from the through-passages generatingthem.

Operation of the System

Operation of the system relies upon positioning at least one skein,preferably a bank, close to a source of sufficient air or gas tomaintain a desirable flux, and, to enable permeate to be collected fromat least one header. A desirable flux is obtained, and provides theappropriate transmembrane pressure differential of the fibers underoperating process conditions.

The transmembrane pressure differential is preferably generated with aconventional non-vacuum pump if the transmembrane pressure differentialis sufficiently low in the range from 0.7 kPa (0.1 psi) to 101 kPa (1bar), provided the pump generates the requisite suction. A pump whichgenerates minimal suction may be used if an adequate “liquid head” isprovided between the surface of the substrate and the point at whichpermeate is withdrawn. Moreover, as explained in greater detail below,once the permeate flow is induced by a pump, the pump may not benecessary, the permeate continuing to flow under a “siphoning effect”.Clearly, operating with fibers subjected to a transmembrane pressuredifferential in the range up to 101 kPa (14.7 psi), a non-vacuum pumpwill provide adequate service in a reservoir which is not pressurized;and, in the range from 101 kPa to about 345 kPa (50 psi), bysuperatmospheric pressure generated by a high liquid head, or, by apressurized reservoir.

A process for separating a permeate from a substrate while maintainingrelatively clean surfaces of fibers in an array, comprises, submerging askein of restrictedly swayable substantially vertical fibers within thesubstrate so that upper and lower end-caps of the skein are mounted oneabove the other with a multiplicity of fibers secured betweencylindrical end-caps, the fibers having their opposed terminal portionspotted in open fluid communication with at least one end-cap; the fibersoperating under a transmembrane pressure differential in the range fromabout 0.7 kPa (0.1 psi) to about 345 kPa (50 psi), and a length from atleast 0.1% to about 2% greater than the direct distance between theopposed upper and lower faces of cured resin in the end-caps, so as topresent, when the fibers are deployed, a generally vertical cylindricalskein of substantially concentrically disposed fibers;

-   maintaining an essentially constant flux substantially the same as    the equilibrium flux initially obtained, indicating that the    surfaces of the fibers are substantially free from further build-up    of deposits once the equilibrium flux is attained;-   collecting the permeate; and,-   withdrawing the permeate.

The foregoing process may be used in the operation of an anaerobic oraerobic biological reactor which has been retrofitted with the membranedevice of this invention. The anaerobic reactor is a closed vessel andthe scrubbing gas is a molecular oxygen-free gas, such as nitrogen.

An aerobic biological reactor may be retrofitted with at least onegas-scrubbed bank of vertical cylindrical skeins, each skein made withfrom 500 to 5000 fibers in the range from 1 m to 3 m long, incombination with a permeate collection means, to operate the reactorwithout being encumbered by the numerous restrictions and limitationsimposed by a secondary clarification system.

Typically, there is no cross flow of substrate across the surface of thefibers in a “dead end” tank. If there is any flow of substrate throughthe skein in a dead end tank, the flow is due to aeration providedbeneath the skein, or to such mechanical mixing as may be employed tomaintain the solids in suspension. There is generally more flow andhigher fluid velocity through the skein in a tank into which substrateis being continuously flowed, but the velocity of fluid across thefibers is generally too insignificant to deter growing microorganismsfrom attaching themselves, or suspended particles, e.g. microscopicsiliceous particles, from being deposited on the surfaces of the fibers.

FIG. 1 presents the results of a comparison of three runs made, oneusing the teachings of Yamamoto in his '89 publication (curve 2), butusing an aerator which introduced air from the side and directed itradially inwards, as is shown in Chiemchaisri et al. A second run (curve1) uses the gas-scrubbed assembly of the '424 patent, and the third run(curve 3) uses a gas-scrubbed skein as described herein except that theheaders were rectangular parallelpipeds. The specific flux obtained withan assembly of an inverted parabolic array with an air distributor means(Yamamoto et al), as disclosed in Wat. Sci. Tech. Vol. 21, Brighton pp43-54, 1989, and, the parabolic array by Cote et al in the '424 patent,are compared to the specific flux obtained with the vertical skein ofthis invention.

The comparison is for the three assemblies having fibers with nominalpore size 0.2 μm with essentially identical bores and surface area in 80L tanks filled with the same activated sludge substrate. The differencesbetween the stated experiment of Yamamoto et al, and that of the '424patent are of record in the '424 patent, and the conditions of thecomparison are incorporated by reference thereto as if fully set forthherein. The vertical skein used herein differs from the '424 skein onlyin the vertical configuration of the 280 fibers, each of which was about1% longer than the distance between the spaced apart headers duringoperation. The flow rate of air for the vertical skein is 1.4 m³/hr/m²using a coarse bubble diffuser.

It will be evident from FIG. 1 in which the specific flux, liters permeter² per hr per unit pressure (conventionally written as (Lmh/kPa), isplotted as a function of operating time for the three assemblies, thatthe curve, identified as reference numeral 3 for the flux for thevertical skein, provides about the same specific flux as the parabolicskein, identified as reference numeral 1. As can be seen, each specificflux readies an equilibrium condition within less than 50 hr, but afterabout 250 hr, it is seen that the specific flux for the invertedparabolic array keeps declining but the other two assemblies reach anequilibrium.

Referring to FIG. 2 there is schematically illustrated, incross-sectional view a vertical cylindrical skein 10, comprising a pairof similar upper and lower end-caps 21 and 22 respectively which serveas cylindrical permeate collection pans. Skein fibers are unsupportedand unattached one to another intermediate the headers; permeate iswithdrawn separately from the upper and lower headers and the draw fromeach combined in a permeate withdrawal manifold. Each “end-cap” has afinished upper/lower header formed directly in it, upper header 23 beingsubstantially identical to lower header 24. Each header is formed bypotting fibers 12 in a potting resin such as a polyurethane or an epoxyof sufficient stiffness to hold and seal the fibers under the conditionsof use. An end-cap was found especially convenient for making relativelysmall surface area skeins because an end-cap for poly(vinyl chloride)“PVC” pipe serves as an excellent header and is commercially readilyavailable; for large surface area skeins, commercially available largerheaders are provided by glass fiber reinforced end-caps for cylindricaltanks. Though the fibers 12 are not shown as close together as theywould normally be, it is essential that the fibers are not in contactwith each other, but that they be spaced apart by the cured resinbetween them. It is also essential that the potting resin adhere to andseal the lower portions 12′ of each of the fibers against leakage offluid under operating conditions of the skein, visual confirmation of aseal is afforded by the peripheries of the fibers being sealed at theupper (fore) and lower (aft) faces 23u and 23b of the upper header 23,and the fore and aft faces 24u and 24b respectively of the lower header24. A conventional finished header may be used in which the ends 12″ ofthe fibers would be flush (in substantially the same plane) as the lowerface 24b. In the best mode, though not visible through an opaqueend-cap, the open ends 12″ of the fibers protrude from the headers'slower (aft or bottom) face 24b.

The finished upper header 23 (fixing lamina) is left adhered to theperiphery of the end-cap 21 when the fugitive lamina is removed througha bore 26 in the upper header; and analogously, the finished Iowa header24 is left adhered to the periphery of the end-cap 22 when the fugitivelamina is removed through a bore 27. The bores 26 and 27 in the upperand lower end-caps have permeate withdrawal tubes 31 and 32,respectively, connected in fluid-tight engagement therein. The permeatetubes 31 and 32, in turn, are connected to a permeate withdrawalmanifold 30.

A detail of a sparger 49 is provided in FIG. 3A The star-shaped sparger40 having radially outwardly extending tubular arms 41 and a centralsupply stub 42, supplies air which is directed into the tubular arms anddischarged into the substrate through air passages 43 in the walls ofthe arms. An air feed tube 44 connected to the central supply stub 42provides air to the sparger 40. The lower end of the central stub 42 isprovided with short projecting nipples 45 the inner ends of which arebrazed to the stub. The outer ends of the nipples are threaded. Thecentral stub and nipples are easy to insert into the center of the massof skein fibers. When centrally positioned, arms 41 which are threadedat one end, are threadedly secured to the outer ends of the nipples.

As illustrated in FIG. 2, lower end-cap 22 rests on the floor F of atank, near a vertical wall W to which is secured a vertical mountingstrut 52 with appropriate fastening means such as a nut 53 and bolt 54.A U-shaped bracket 51 extends laterally from the base of the mountingstrut 52. The arms of the U-shaped bracket support the periphery ofupper end-cap 21, and to ensure that the end-cap stays in position, itis secured to the U-shaped bracket with a right angle bracket andfastening means (not shown). A slot in mounting strut 52 permits theU-shaped bracket to be raised or lowered so that the desired distancebetween the opposed faces 23b and 24u of the upper and lower headersrespectively is less than the length of any potted fiber, measuredbetween those faces, by a desired amount. Adjustability is particularlydesirable if the length of the fibers tends to change during service.

Instead of withdrawing permeate through both tubes 31 and 32 it may bedesirable to withdraw permeate from both the upper 21 and lower end-capsthrough only the upper tube 31. If it is, a permeate connector tube 33(shown in phantom outline), is inserted within the mass of skein fibers12 through the headers 23 and 24, connecting the permeate collectionzone 29 in the lower end-cap in open fluid communication with thepermeate collection zone 28 in the upper end-cap; and, bore 27 isplugged.

As illustrated in FIG. 3, in the event that withdrawal of permeate fromthe upper permeate collection zone 28 alone is sufficient, and it isunnecessary to withdraw permeate from both the upper and lower zones 28and 29, the lower bore 27 of the lower end-cap 22 is simply plugged witha plug 25. Since, under such circumstances, it does not matter if thelower ends 12″ of the fibers are plugged, and permeate collection zone29 serves no essential function, the zone 29 may be filled with pottingresin.

The step-wise procedure for forming an array to be potted in the novelheader is described with respect to an array “A” illustrated in FIG. 4,as follows:

A desired number of fibers 12 are each cut to about the same length witha sharp blade so as to leave both opposed ends of each fiber with anessentially circular cross-section. The fibers are coplanarly disposedside-by-side in a linear array on a flexible planar support means suchas strips or cards 15 and 16 which can be formed into a loose roll.Preferably strips of stiff paper are coated with an adhesive, e.g. acommercially available polyethylene hot-melt adhesive, so that thefibers are glued to the strips and opposed terminal portions 12″respectively of the fibers, extend beyond the strips. The stripssecuring the fibers extend over only the intermediate portions 12′ ofthe fibers. Alternatively, to avoid gluing fibers to the strips,flexible strips of an elastomeric material such as a 50-90 Shore Apolyurethane having pre-formed parallel spaced-apart grooves thereininto which the opposed ends of fibers may be snugly held, can be used.

Referring to FIG. 5 there is schematically illustrated the position of aspiral roll of an array of fibers which roll is secured with a rubberband 18 or other clamping means as it is held for potting in ring 20which is clamped (not shown) tightly to a flat plate 14 so as to sealthe periphery of ring 20 against the plate. The thickness of a stripand/or adhesive is sufficient to ensure that the fibers in successivelayers of the roll are kept spaced apart. Preferably, this thickness isabout the same as, or relatively smaller than the outside diameter of afiber, preferably from about 0.5d to 1d thick, which becomes the spacingbetween adjacent outside surfaces of fibers in successive layers of thespiral. FIG. 2A illustrates the spiral pattern of openings in the ends12″ of the fibers, obtained when the spiral roll is potted in a pottingring 20.

In another embodiment a series of successively larger diameter circulararrays may be formed, each a small predetermined amount larger than thepreceding one, and the arrays secured, preferably adhesively, one to thenext, near their upper and lower peripheries respectively to form adense cylindrical mass of fibers. In such a mass of fibers, each arrayis secured both to a contiguous array having a next smaller diameter, aswell as to a contiguous array having a next larger diameter, except forthe innermost and outermost arrays which have the smallest and largestdiameter, respectively. After the nested arrays are potted in ring 20,the resulting pattern of concentric circles formed by the open lowerends 12″ of the fibers in the lower face 24b of the lower header isillustrated in FIG. 2B.

To make a skein with plural arrays arranged chord-like within a ring 20or resin-confining means, plural planar arrays are formed on pairs ofstrips, each having a length corresponding to its position as a chordwithin a potting ring in which the skein fibers are to be potted. Thatis, each array is formed on strips of diminishing width, measured fromthe central array which is formed on a strip having a width slightlyless than the inner diameter of the ring 20 in which the stack is to bepotted. The arrays are stacked within the ring, the widest arraycorresponding in position to the diameter of the ring. For a chosenfiber, the larger the surface area required in a skein, the greater thenumber of fibers in each array, the bigger the diameter of the ring, andthe wider each chord-like array. The plural arrays are preferablyadhered one to the other by coating the surfaces of fibers with adhesiveprior to placing a strip of the successive array on the fibers.Alternatively, the stacked arrays may be held with a rubber band beforebeing inserted in the potting ring. The resulting chord-like pattern ofthe open lower ends 12″ of the fibers in the lower face 24b of the lowerheader is illustrated in FIG. 2C. Ease of handling and the desireddensity of fibers per unit area of header will normally determine thechoice of an embodiment for forming the potted skein.

Referring further to FIG. 5, the ring 20 serves the function of pottingpan for forming the upper and lower headers. After the skein fibers arepotted in finished headers, and the fibers checked for leaks so that anyindividual defective fiber may be plugged, the ring is snugly held in anend-cap (not shown) which serves as a permeate collection pan. The ring20 may be adhesively secured in the end-cap or is held in fluid-tightengagement with it using a circumferential gasket. Whether the stripsseparating successive rows of fibers are to be retained will determinethe depth L1 or L1′ of fugitive header. The header of fixing material(thickness L1-L2) may have a cushioning layer (thickness L2-L3). If agasketing lamina is desired, when the header is to be secured above apermeate pan, a liquid gasketing material is poured and cured over thefugitive lamina to provide the gasketing lamina of thickness L1-L4.

The description of the method of forming a header is detailed in the'119 parent application, and in the '921 provisional application, whichdescription is incorporated by reference thereto as if fully set forthherein.

The restricted swayability of the fibers generates some intermittent‘snapping’ motion of the fibers which may break the potted fibers aroundtheir circumferences, at the interface of the fore face and substrate.To combat such damage, the fixing material is preferably chosen so as toprovide adequate cushioning of the fibers at the interface. Such amaterial is typically an elastomer having a hardness in the range from50 Shore A to about 20 Shore D.

Where a chosen fixing material is so hard as to cause the aforesaiddamage, it is minimized by providing an additional lamina of materialwhich is softer than the fixing lamina, to serve as a cushioning lamina.Such a cushioning lamina is formed integrally with the fixing lamina, bypouring cushioning liquid (so termed for its function when cured) overthe fixing lamina to a desired depth sufficient to provide enough ‘give’around the circumferences of the fibers to minimize the risk ofshearing. Such cushioning liquid, when cured is rubbery, having ahardness in the range from about Shore A 30 to Shore D 20, and ispreferably a polyurethane or silicone or other elastomeric materialwhich will adhere to the fixing lamina. Upon removal of the fugitivelamina, the finished header thus formed has the combined thicknesses ofthe fixing lamina and the cushioning lamina, when the strips 15 are cutaway.

As illustrated in FIGS. 2 and 3, a finished integral header may bedirectly formed in end-caps 21 and 22 into which permeate is to flow,thus solving the problem of sealing a conventionally formed and demoldedheader in a permeate pan.

Referring to FIG. 6 there is schematically illustrated a skein 60 withconventionally formed and demolded upper and lower headers 63 and 64respectively potting the terminal portions of fibers 12. Each header isformed as described in U.S. Pat. No. 5,202,023. The ends of fibers in anarray held in a spiral roll with a rubber band are dipped in resin orpaint to prevent resin penetration into the bores of the fibers duringthe potting process. The ends of the roll are then placed in a mold anduncured resin added to saturate the ends of the fibers and fill thespaces between the individual fibers in the roll. The cured, molded endsare removed from the molds and the molded ends cut off (see, bridgingcols 11 and 12).

Upper header 63 is placed against the lip 67 of a stainless steelpermeate pan 61 and sealed in it with a peripheral gasket 65 placedcircumferentially between the vertical wall of the header 23 and thevertical peripheral surface of the wall 66 of the permeate pan;alternatively, as illustrated in lower permeate pan 62, the gasket 65may be placed between the lower peripheral surface of the lower header64 and the peripheral lip 67 on which the header rests. A suitablesealing gasket or sealing compound typically used is a polyurethane orsilicone resin. The periphery of each header is secured to itsrespective permeate pan with screws or other suitable fastening means toensure a fluid-tight seal. The strips on which the array of fibers washeld prior to being potted remain in the header, though not shown in theFigure.

As seen, the open ends of the embedded terminal portions 12′ of thefibers are in the same plane as the lower face of the header 11 becausethe fibers are conventionally potted and the header sectioned to exposethe open ends. In this prior art method, sectioning the mold unavoidablydamages at least some, and typically, a substantial number of theembedded fibers. Permeate connector tube is press-fitted when it isinserted in through-bores in the upper and lower headers after they aredemolded and the plugged ends of the fibers cut away. As before, theskein is provided with a sparger 40 supplied by a flexible air supplytube 44 and the lower permeate pan 62 rests on floor F of a tank. Theupper permeate pan rests on a U-shaped bracket 51 positioned so as toprovide the desired slack in the fibers 12.

In the most preferred embodiment the novel method of potting disclosedherein is used. Because this method denies ready access to the ends ofthe fibers once finished headers are formed within a ring or an end-cap,the ends of the fibers protrude from the lower face 24b of the lowerheader 24 and the upper face 23u of the upper header 23 into therespective permeate collection zones.

Referring to FIG. 7 there is illustrated a skein 70 with upper and lowerend-caps in which are sealed upper and lower ring headers formed inupper and lower rings 20u and 29b respectively, after the fibers in theskein are tested to determine if any is defective. Before an array isrolled into a spiral, as before, a sparger 40 with a rigid air-supplytube 45 is placed in the array so that upon forming a spiral roll theair-supply tube is centrally axially held within the roll. The lower endof the roll is then potted forming a lower finished header 74 in whichthe lower end 46 of the air-supply tube is potted, fixing the positionof the arms 41 of the sparger just above the upper face 74u of theheader 74.

In an analogous manner, an upper header 73 is formed in ring 20u and apermeate connector tube 33 press-fitted into aligned through-bores inthe upper and lower headers. Upper end 47 of air-supply tube 45 isinserted through an axial bore 48 within upper end-cap 71 which isslipped over the ring 20u the outer periphery of which is coated with asuitable adhesive, to seal the ring 20u in the end-cap 71. The peripheryof the upper end 47 is sealed in the end cap 71 with any conventionalsealing compound.

Referring to FIG. 8 there is schematically illustrated anotherembodiment of a skein 80 in which rigid permeate tube 85 is heldconcentrically within a rigid air-supply tube 86 which is potted axiallywithin skein fibers 12 held between opposed upper and lower headers 83and 84 in upper and lower rings 20u and 20b which are in turn sealed inend-caps 81 and 82 respectively. For ease of manufacture, the lower end85b of permeate tube 85 is snugly fitted and sealed in a bushing 87. Thebushing 87 and end 85b are then inserted in the lower end 86b of the airsupply tube 86 and sealed in it so that the annular zone between theouter surface of permeate tube 85 and the inner surface of air supplytube 86 will duct air to the base of the fibers but not permit permeateto enter the annular zone. The air supply tube is then placed on anarray and the array is rolled into a spiral which is held at each endwith rubber bands. The lower end of the roll is placed in a ring 20b anda lower ring header is formed with a finished header 84 as describedabove. It is preferred to use a relatively stiff elastomer having ahardness in the range from 50 Shore A to about 20 Shore D, and mostpreferred to use a polyurethane having a hardness in the range from 50Shore A to about 20 Shore D, measured as set forth in ASTM D-790, suchas PTU-921 available from Canadian Poly-Tech Systems. To form the upperfinished header 83 the air supply tube is snugly inserted through anO-ring held in a central bore in a plate such as used in FIG. 5, toavoid loss of potting resin from the ring 20, and the fugitive resin andfinishing resins poured and cured, first one then the other, in thering. Lower finished header 84 is formed with intermediate portions 12b′embedded, and terminal portions 12b″ protruding from the header's aftface. Upper finished header 83 is formed with intermediate portions 12u′embedded, and terminal portion 12u″ protruding from the header's foreface. After the finished headers 83 and 84 are formed and the fiberschecked for defects, the upper end 86u of the air supply tube 86 isinserted through a central bore 88 in upper end-cap 81 and sealed withinthe bore with sealing compound or a collar 89. Preferably the permeatetube 85, the air supply tube 86 and the collar 89 are all made of PVC sothat they are easily cemented together to make leak-proof connections.

As shown, permeate may be withdrawn through the permeate tube 85 fromthe permeate collection zone in the lower end-cap 82, and separatelyfrom the upper end-cap 81 through permeate withdrawal port 81p which maybe threaded for attaching a pipe fitting. Alternatively, the permeateport 81p may be plugged and permeate withdrawn from both end-capsthrough the permeate tube 85.

Upper end 85u of permeate tube 85 and upper end 86u of air supply tube86 are inserted through a T-fitting 101 through which air is supplied tothe air supply tube 86. The lower end 101b of one of the arms of the T101 is slip-fitted and sealed around the air supply tube. The upper end101u of the other arm is inserted in a reducing bushing 102 and sealedaround the permeate tube. Air supplied to intake 103 of the T 101travels down the annular zone between the permeate tube and the airsupply tube and exits through opposed ports 104 in the lower portion ofthe air supply tube, just above the upper face 84u of the lower header84. It is preferred to thread ports 104 to threadedly secure the ends ofarms 41 to form a sparger which distributes air substantially uniformlyacross and above the surface 84u. Additional ports may be provided alongthe length of the vertical air supply tube, if desired.

Referring to FIG. 9 there is shown a bank 110 of a pair of side-by-sideskeins 111 and 112 substantially identical to skein 80 shown in FIG. 8,mounted in substrate against a wall W of a tank. The permeate withdrawaltubes 85 are manifolded to a common permeate manifold 135 and theT-fittings 101 for the air supply tubes 86 are manifolded to a commonair supply 145. Permeate withdrawal tubes 131 are manifolded to aseparate manifold 155 to provide greater flexibility than if manifoldedwith withdrawal tubes 85, and also to permit flushing the skein fibers.All connections to conduits to the bank are made to the upper end-capsfor ease of operation. A skein with relatively low surface area may haveas few as 100 fibers, while a skein with relatively large surface aremay have as many as 2500 fibers, or more.

When permeate is withdrawn in the same plane as the permeate withdrawalmanifold, and the transmembrane pressure differential of the fibers isin the range from 35-75 kPa (5-10 psi), the manifold may be connected tothe suction side of a centrifugal pump which will provide adequate NPSH.

In general, it is preferred to withdraw permeate from both the upper andlower headers, until the flux declines to so low a level as to requirethat the fibers be backwashed or backflushed. The skeins may bebackwashed by introducing a backwashing fluid through the permeatemanifold under sufficient pressure to force the fluid through the poresof the membranes. This may be done in a skein having the configurationshown in FIGS. 2, 3, 6 and 7. The fibers may be backflushed in skeins111 and 112 in the same manner as the skein shown in FIG. 8.Backflushing fluid is introduced through one permeate tube (as forexample one connected to permeate port 81p) and removed through theother permeate tube (85). The skeins 111 and 112 may also be backwashedby blocking flow of permeate through one manifold and pressuringbackwashing fluid through the other permeate manifold.

Referring to FIG. 10, there is schematically illustrated anotherembodiment of an assembly, referred to as a “stand-alone” bank orcluster 120 of skeins, four of which are referenced by numerals 121,122, 123 and 124. The cluster is referred to as being a “stand-alone”because the spacer means between end-caps is provided by the concentricair-supply and permeate tubes potted in the headers. A cluster isusually used when mounting the skeins against the wall of a reservoir isless effective than placing the cluster in spaced-apart relationshipfrom a wall of the tank. In other respects, the cluster 120 is analogousto the wall-mounted bank 110 illustrated in FIG. 9. As will now beevident, the number of skeins connected in a cluster is limited only bythe connections provided to manifold the skeins adequately.

In the best mode illustrated, each upper end-cap is provided with rigidPVC tubular nipples adapted to be coupled with fittings such as ells andtees to the appropriate manifolds.

In another embodiment of the invention, a bioreactor is retrofitted withplural banks of skeins schematically illustrated in the elevational viewshown in FIG. 11, and the plan view shown in FIG. 12. The clarifier tankis a large circular tank 90 provided with a vertical, circular outerbaffle 91, a vertical circular inner baffle 92, and a bottom 93 whichslopes towards the center (apex) for drainage of accumulating sludge.Alternatively, the baffles may be individual, closely spaced rectangularplates arranged in outer and inner circles, but continuous cylindricalbaffles (shown) are preferred. Irrespective of which baffles are used,the baffles are located so that their bottom peripheries are located ata chosen vertical distance above the bottom. Feed is introduced throughfeed line 94 in the bottom of the tank 90 until the level of thesubstrate rises above the outer baffle 91.

A bank 210 of plural side-by-side skeins, analogous to those in the bank110 depicted in FIG. 9, is deployed against the periphery of the innerwall of a bioreactor 90 with suitable mounting means in an outer annularpermeate extraction zone 95′ (FIG. 12) formed between the circular outerbaffle 91 and the wall of the tank 90, at a depth sufficient to submergethe fibers. A clarification zone 91′ is defined between the outercircular baffle 91 and inner circular baffle 92. The inner circularbaffle 92 provides a vertical axial passage 92′ through which substrateis fed into the tank 90. The side-by-side skeins form a dense curtain offibers hanging vertically between upper 81 and lower 82 end-caps.Permeate is withdrawn through permeate manifolds 135 and 155 and air isintroduced through air-manifold 145, extending along the inner wall ofthe tank and branching out with connections to adjacent end-caps.Because air is sparged between fibers in such a manner as to havebubbles contact essentially the entire surface of each fiber which iscontinuously awash with bubbles while the fibers are vertical, the airis in contact with the surfaces of the fibers longer than if they werearcuate, and the air is used most effectively to maintain a high fluxfor a longer period of time than would otherwise be maintained.

It will be evident that if the tank is at ground level, there will beinsufficient liquid head to induce a desirable liquid head under gravityalone. Without an adequate siphoning effect, a centrifugal pump may beused to produce the necessary suction. Such a pump should be capable ofrunning dry for a short period, and of maintaining a vacuum on thesuction side of from 25.5 cm (10″)−51 cm (20″) of Hg, or −35 kPa (−5psi) to −70 kPa (−10 psi). Examples of such pumps rated at 18.9 L/min (5gpm) @ 15″Hg, are (i) flexible-impeller centrifugal pumps, e.g. Jabsco#30510-2003; (ii) air operated diaphragm pumps, e.g. Wilden M2; (iii)progressing cavity pumps, e.g. Ramoy 3561; and (iv) hosepumps. e.g.Waukesha SP 25.

EXAMPLE 1

Microfiltration of an activated sludge at 30° C. having a concentrationof 25 g/L (2.5% TSS) is carried out with a cylindrical skein ofpolysulfone fibers in a pilot plant tank. The fibers are “air scrubbed”at a flow rate of 12 CFM (0.34 m³/min) with a coarse bubble diffusergenerating bubbles in the range from about 5 mm to 25 mm in nominaldiameter. The air is sufficient not only for the oxidation requirementsof the biomass but also for adequate scrubbing. The fibers have anoutside diameter of 1.7 mm, a wall thickness of about 0.5 mm, and asurface porosity in the range from about 20% to 40% with pores about 0.2μm in diameter, both latter physical properties being determined by amolecular weight cut off at 200,000 Daltons. The skein which has 1440fibers with a surface area of 12 m² is wall-mounted in the tank, thevertical spaced apart distance of the headers being about 1% less thanthe length of a fiber in the skein. The opposed ends of the fibers arepotted in upper and lower headers respectively, each about 41 cm longand 10 cm wide. The fixing material of the headers is a polyurethanehaving a hardness in the range from 50-90 Shore A The averagetransmembrane pressure differential is about 34.5 kPa (5 psi). Permeateis withdrawn through a conduit connected to a pump generating about 34.5kPa (5 psi) suction. Permeate is withdrawn at a specific flux of about0.7 lm²h/kPa yielding about 4.8 l/min of permeate which has an averageturbidity of <0.8 NTU, which is a turbidity not discernible to the nakedeye.

EXAMPLE 2

Comparison of Operation of a Vertical Skein (ZW 72) in DifferentOrientations

In the following comparison, three pairs of identical skeins withequally slack fibers are variously positioned (as designated) aboveaerators in a bioreactor. Each pair is subjected to the same dischargeof air through identical aerators. Rectangular but not square headersare chosen to determine whether there is a difference between each oftwo flat horizontal orientations, which difference would not exist in ahorizontal skein with cylindrical headers. A pair of identicalrectangular skeins, each having headers 41.66 cm (16.4 in) in length(x-axis), 10.16 cm (4 in) in width (y-axis) and 7.62 cm (3 in) in height(z-axis), in which are potted 1296 Zenon® MF200 microfiltration fiberspresenting a nominal fiber surface area of 6.25 m², were tested in threedifferent orientations in a bioreactor treating domestic wastewaters.The fibers used are the same as those used in Example 1 above. Thedistance between opposed faces of headers is 90 cm (35.4 in) which isabout 2% less than the length of each fiber potted in those headers.

In a first test, the two (first and second) skeins were stackedlaterally, each in the same direction along the longitudinal axis, witha 2.5 cm (1 in) thick spacer between the headers, the headers of eachskein being in a horizontal flat orientation (area 41.66 cm×7.62 cm) isspaced apart 7.62 cm (3 in) above the floor on which lies the aeratorsin the form of three side-by-side linear tubes with 3 mm (0.125″)openings. The first skein which is directly above the aerators istherefore referred to as the “lower skein”.

In a second test, the same first and second skeins are each rotated 90°about the longitudinal x-axis and placed contiguously one beside theother. In this “horizontal 90°” orientation (area defined by 10.16cm×7.62 cm) is spaced apart from the aerators as in the prior test.

In a third test, the first and second skeins are placed side-by-side invertical orientations and aeration is provided with a rectangular tubearound the periphery of the skein, with perforations in the tube, andthere is no internal aerator.

Each test provides the fibers in each orientation with the identicalamount of air. Permeate was withdrawn with a pump with a NPSH of 0.3 bar(10″ of Hg). The conditions were held constant until it was observedthat the flux obtained for each test was substantially constant, thisbeing the equilibrium value. After this occurred, each skein was backpulsed for 30 sec with permeate every 5 minutes to maintain the flux atthe equilibrium value.

The test conditions for each of the above three runs were as follows:

-   TSS in bioreactor 8 g/L; Temperature of biomass 19° C.-   Flow rate of air 0.2124 m³/min/skein; Suction on fibers 25.4 cm of    Hg

FIG. 13 is a bar graph which shows the average flux over a 24 hr periodfor each orientation of the skein as follows:

Orientation Average flux L/m²/hr over 24 hr Horizontal flat 21.2 LMHHorizontal 90° 17.8 LMH Vertical 27.7 LMHThis conclusively demonstrates that the vertical orientation of theskein fibers produces the highest overall flux.

EXAMPLE 3

Comparison of Positions of Aerator Inside and Outside the Skein Fibers

In this test the difference in flux is measured in a bioreactor treatingwastewater contaminated with ethylene glycol, the difference dependingupon how a single cylindrical vertical skein (ZW 172) having a nominalsurface area of 16 m² is aerated with 3.5 L/min (7.5 scfm). The skein isformed as shown in FIG. 16 around a central PVC pipe having an o.d. of7.5 cm, the fibers being disposed in an annular zone around the centralsupport, the radial width of the annular zone being about 7.5 cm, sothat the o.d. of the skein is about 11.25 cm.

In a first test, air is introduced within the skein; in a second test,air is introduced around the periphery of the skein. After equilibriumis reached, operation is typically continued by back pulsing the skeinwith permeate at chosen intervals of time, the interval depending uponhow quickly the fibers foul sufficiently to decrease the fluxsubstantially.

The process conditions, which were held constant over the period of thetest, were as follows:

TSS 17 g/L; Temperature of biomass 10.5° C. Flow rate of 0.2124 m³/min;Suction on fibers 25.4 cm of Hg airFor external aeration

A perforated flexible tube with holes about 3 mm in diameter spacedabout 2.5 cm apart was wrapped around the base of the ZW 72 skein andoriented so that air is discharged in a horizontal plane, so thatbubbles enter laterally into the skein, between fibers. Thereafter thebubbles rise vertically through the skein fibers. Lateral dischargehelps keep the holes from plugging prematurely.

For internal aeration

The central tubular support was used as the central air distributionmanifold to duct air into five 4″ lengths of ¼″ pipe with ⅛″ holes at 1″intervals, plugged at one end, in open flow communication with thecentral pipe, forming a spoke-like sparger within the skein, at thebase. The number of holes is about the same as the number in theexternal aerator, and the flow rate of air is the same. As before theholes discharge the air laterally within the skein, and the air bubblesrise vertically within the skein, and exit the skein below the upperheader.

FIG. 14 is a plot of flux as a function of time, until the flux reachesan equilibrium value. Thereafter the flux may be maintained by backpulsing at regular intervals. As is evident, the equilibrium flux withexternal aeration is about 2.6 LMH, while the flux with internalaeration is about 9.9 LMH which is nearly a four-fold improvement. Fromthe foregoing it will be evident that since it is well-known that fluxis a function of the flow rate of air, all other conditions being thesame during normal operation, a higher flux is obtained with internalaeration with the same flow of air.

EXAMPLE 4

Comparison of skeins in which one has swayable fibers, the other doesnot

The slackness in the fibers is adjusted by decreasing the distancebetween headers. Essentially no slack is present (fibers are taut) whenthe headers are spaced at a distance which is the same as the length ofa fiber between its opposed potted ends. A single ZW 72 skein is usedhaving a nominal surface area of 6.7 m² is used in each test, in abioreactor to treat wastewater contaminated with ethylene glycol.Aeration is provided as shown in FIG. 9 (no internal aeration) withlateral discharge of air bubbles into the skein fibers through whichbubbles rose to the top.

In the first test the headers are vertically spaced apart so that thefibers are taut and could not sway.

In the second test, the headers were brought closer by 2 cm causing a2.5% slackness in each fiber, permitting the slack fibers to sway.

As before the process conditions, which were held constant over theperiod of the test, were as follows:

Suspended solids 17 g/L Temperature of biomass 10.5° C. Flow rate of air0.2124 Suction on fibers 25.4 cm of Hg m³/min;

FIG. 15 is a plot of flux as a function of time, until the flux reachesan equilibrium value. Thereafter the flux may be maintained by backpulsing at regular intervals as before in example 1. As is evident, theequilibrium flux with no swayability is about 11.5 LMH, while the fluxwith 2.5% slack is about 15.2 LMH, which is about a 30% improvement.

EXAMPLE 5

Filtration of water with a vertical cylindrical skein to obtain clarity

A cylindrical skein is constructed as in FIG. 16 with Zenon® MF200fibers 180 cm long, which provide a surface area of 25 m² in cylindricalheaders having a diameter of 28 cm held in end-caps having an o.d. of 30cm. Aeration is provided with a spider having perforated cross-arms with3 mm (0.125″) dia. openings which discharge about 10 liter/rain (20scfm, standard ft³/min) of air. This skein is used in four typicalapplications, the results of which are provided below. In each case,permeate is withdrawn with a centrifugal pump having a NPSH of about 0.3bar (10″ Hg), and after equilibrium is reached, the skein is backflushedfor 30 sec with permeate every 30 min.

A. Filtration of Surface (Pond) Water having 10 mg/L TSS

-   Result—permeate having 0.0 mg/L TSS is withdrawn at a rate of 2000    liters/hr (LPH) with a turbidity of 0.1 NTU. A “5 log” reduction    (reduction of original concentration by five orders of magnitude) of    bacteria, algae, giardia and cryptosporidium may be obtained, thus    providing potable water.    B. Filtration of Raw Sewage with 100 mg/L TSS-   Result—permeate having 0.0 mg/L suspended solids is withdrawn at a    rate of 1000 LPH (liters/hr) with a turbidity of 0.2 NTU. Plural    such skeins may be used in a bank in the fully scale treatment of    industrial wastewater.    C. Filtration of a mineral suspension containing 1000 mg/L TSS of    iron oxide particles-   Result—permeate having 0.0 mg/L suspended solids is withdrawn at a    rate of 3000 LPH (liters/hr) with a turbidity of 0.1 NTU. High flux    is maintained with industrial wastewater containing mineral    particles.    D. Filtration of fermentation broth with 10,000 mg/L bacterial cells-   Result—permeate having 0.0 mg/L suspended solids is withdrawn at a    rate of 1000 LPH (liters/hr) with a turbidity of 0.1 NTU. The broth    with a high biomass concentration is filtered non-destructively to    yield the desired permeate, as well as to save living cells for    reuse.

EXAMPLE 6

Special Purpose Mini-Skein

The following examples illustrate the use of a mini-skein for typicalspecific uses such as filtration of (i) raw sewage to obtain solids-freewater samples for colorimetric analyses, (ii) surface water for use in arecreational vehicle (“camper”) or motor home, or (ii) water from asmall aquarium for fish or other marine animals.

A cylindrical mini-skein is constructed as shown in FIG. 16, withcylindrical headers having an o.d. of 5 cm (2″) and a thickness of 2 cm(0.75″) with 30 fibers, each 60 cm long to provide a surface area of 0.1m². The skein is mounted on a base on which is also removably disposed ablower to discharge 15 L/min of air at 12 kPa (3 psig) through a spargerwhich has 1.6 mm (0.0625″) openings, the air flowing through the skeinupwards along the fibers. Also removably mounted on the base is aperistaltic pump which produces a vacuum of 0.3 bar (10″ Hg). In eachapplication, the self-contained skein with integral permeate pump andgas-discharge means, is placed, for operation, in a cylindricalcontainer of the substrate to be filtered.

The results with each application (A)-(D) are listed below:

-   (i) Raw sewage contains 100 mg/L TSS; permeate containing 0.0 mg/L    TSS having a turbidity of 0.2 NTU, is withdrawn at 0.1 LPH.-   (ii) Aquarium water withdrawn contains 20 mg/L TSS, including algae,    bacteria, fungus and fecal dendritus; permeate containing 0.0 mg/L    TSS having a turbidity of 0.2 NTU, is withdrawn at 0.1 LPH.-   (iii) Pond water withdrawn contains 10 mg/L TSS; permeate containing    0.0 mg/L TSS having a turbidity of 0.2 NTU, is withdrawn at 0.1 LPH.

GLOSSARY

The following glossary is provided for terms in the approximate order inwhich they are used in the specification to define their meaning in thecontext in which they are used.

“array”—plural, essentially vertical fibers of substantially equallengths, the one ends of each of which fibers are closely spaced-apart,either linearly in the transverse (y-axis herein) direction to provideat least one row, and typically plural rows of equidistantly spacedapart fibers. Less preferably, a multiplicity of fibers may be spaced ina random pattern. The opposed ends of fibers are sealed in opposedheaders so that substrate does not contaminate permeate in permeatecollection means in which the headers are peripherally sealed.

“bundle”—plural elements held together, e.g. plural arrays which may bea stack of planar arrays, or arcuate or circular arrays, or a rolledspiral.

“bank”—used for brevity, to refer to a bank of skeins; in the bank, arow (or other configuration) of lower headers is directly beneath a rowof upper headers.

“cylindrical skein”—a vertical skein in which the permeate collectionmeans has a cylindrical configuration.

“dead end tank”—a tank or bioreactor from which no liquid other than thepermeate is removed.

“fibers”—used for brevity to refer to hollow fiber membranes.

“flux”—unit flow (liters/hr), through a membrane of unit surface area(meter²), flux is given as Lm²h or LMH.

“fugitive material”—material which is either (i) soluble in a medium inwhich the fibers and fixing material are not soluble, or (ii)fluidizable by virtue of having a melting point (if the material iscrystalline) below that which might damage the fibers or fixingmaterial; or, the material has a glass transition temperature Tg (if thematerial is non-crystalline), below that which might damage the fibersor material(s) forming the non-fugitive header; or (iii) both solubleand fluidizable.

“header”—a solid body in which one of the terminal end portions of eachone of a multiplicity of fibers in the skein, is sealingly secured topreclude substrate from contaminating the permeate in the lumens of thefibers. The body is of arbitrary dimensions formed from a natural orsynthetic resinous material (thermoplastic or thermosetting).

“integral header”—combination of header and permeate collection means,in which combination the header is peripherally sealed in fluid-tightrelationship with the permeate collection means.

“integral single skein”—a skein in an integral finished header is formedin the permeate pan or end-cap, sealing the header therein.

“mini-skein”—a self-contained gas-scrubbed assembly of a skein having asurface area less man about 5 m², in combination with an integrallypackaged gas blower and permeate pump.

“multicomponent liquid feed”—fruit juices to be clarified orconcentrated; wastewater or water containing particulate matter;proteinaceous liquid dairy products such as cheese whey, and the like.

“non-vacuum pump”—generates a net suction side pressure difference, or,net positive suction head (NPSH), adequate to provide the transmembranepressure differential generated under the operating conditions; may be acentrifugal, rotary, crossflow, flow-through, or other type.

“permeability”—flux per unit pressure, Lm²h/kPa; sometimes referred toas specific flux.

“permeate collection means”—receptacle beneath a header in whichreceptacle permeate collects.

“ring header”—header having a cylindrical shape.

“rectangular skein”—a vertical skein in which the permeate collectionmeans has a configuration of a rectangular parallelpiped.

“skein”—used for brevity to refer to either a cylindrical skein or avertical skein, or both, having plural arrays potted in opposed headers,the fibers having a critically defined length relative to the verticaldistance between headers of the skein. The defined length limits theside-to-side movement of the fibers in the substrate in which they aredeployed, except near the headers where there is negligible movement.

“skein fibers”—fibers which make up the cylindrical skein

“vertical skein”—an integrated combination of structural elementsincluding (i) a multiplicity of vertical fibers of substantially equallength; (ii) a pair of headers in each of which are potted the opposedterminal portions of the fibers so as to leave their ends open; and,(iii) permeate collection means held peripherally in fluid-tightengagement with each header so as to collect permeate from the ends ofthe fibers.

“substrate”—multicomponent liquid feed.

“particulate matter”—micron-sized (from 1 to about 44 μcm) andsub-micron sized (from about 0.1 μm to 1 μm) filtrable matter whichincludes not only particulate inorganic matter, but also dead and livebiologically active microorganisms, colloidal dispersions, solutions oflarge organic molecules such as fulvic acid and humic acid, and oilemulsions.

“restrictedly swayable”—the extent to which fibers may sway in a zone ofconfinement, which extent is determined by the free length of the fibersrelative to the fixedly spaced-apart headers, and the turbulence of thesubstrate.

“stack of arrays”—plural rows of arrays, which are densely packed toform, after they are potted, a skein.

“substantially concentrically”—describes a configuration in which inwhich individual fibers are either vertical and spaced apart along thecircumference of a circle drawn about the central vertical axis, or,spirally disposed, successive layers of the fibers typically beingclosely spaced-apart in the x-y plane, not only radially outwards fromthe central axis, but also along the spiral in that plane so that theyappear to be concentrically distributed at successively increasingradial distances from the central axis.

“transmembrane pressure differential”—pressure difference across amembrane wall, resulting from the process conditions under which themembrane is operating.

“unsupported”—not supported except for spacer means to space theheaders.

“vacuum pump”—capable of generating a suction of at least 75 cm of Hg.

“zone of confinement” (or “bubble zone”)—a zone through which bubblesrise along the outer surfaces of the fibers. The bubble zone, in turn,is determined by one or more columns of vertically rising gas bubblesgenerated near the base of a skein.

1. In a gas-scrubbed assembly comprising, a microfiltration membranedevice in combination with a gas-distribution means to minimize build-upof particulate deposits on the surfaces of hollow fiber membranes(“fibers”) in said device, and to recover permeate from a multicomponentliquid substrate while leaving particulate matter therein, said membranedevice comprising, a multiplicity of fibers, unconfined in a shell of amodule, said fibers being swayable in said substrate, said fibers beingsubject to a transmembrane pressure differential in the range from about0.7 kPa (0.1 psi) to about 345 kPa (50 psi); a first and second headerdisposed in transversely spaced-apart relationship within saidsubstrate, each header being formed with a potting resin cured in aresin-confining means; said first header and second header havingopposed terminal end portions of each fiber sealingly secured therein,all open ends of said fibers extending from a permeate-discharging faceof at least one header; permeate collection means to collect saidpermeate through at least one of said headers sealingly connected inopen fluid communication with permeate-discharging faces of saidheaders; means for withdrawing said permeate; and, said gas-distributionmeans is located within a zone beneath said skein, said gas-distributionmeans having through-passages therein adapted to have sufficient gasflowed therethrough to generate enough bubbles flowing in a column ofrising bubbles between and around said skein fibers, to keep surfaces ofsaid fibers awash in bubbles; said fibers, said headers and saidpermeate collection means together forming a vertical cylindrical skeinwherein said fibers are essentially vertically disposed; said firstheader being upper and disposed in vertically spaced-apart relationshipabove said second header with opposed faces of said headers at a fixeddistance, said fibers being substantially concentrically disposedrelative to the vertical axis between said headers; each of said fibershaving substantially the same length, said length being from at least0.1% greater, to less than 5% greater than said fixed distance so as topermit restricted displacement of an intermediate portion of each fiber,independently of the movement of another fiber; the improvementcomprising, each said header having said fibers spaced apart by aflexible support means having a thickness corresponding to a desiredlateral spacing between adjacent fibers, said support means extendingover only each terminal portion of said fibers near their ends, so as tomaintain said ends in closely-spaced apart relationship, said gasdistribution means being disposed between said fibers and havingthrough-passages adapted to discharge said bubbles which rise verticallysubstantially parallel to, and in contact with said fibers, movement ofwhich is restricted within said column; whereby said permeate isessentially continuously withdrawn.
 2. The gas-scrubbed assembly ofclaim 1 wherein, said restricted displacement is in the lateral orhorizontal direction, said headers are non-removably formed within saidresin-confining means, and, said gas-distribution means includes anaerator means disposed adjacent to said lower header's upper facedischarging said gas in an amount in the range from 0.47-14 cm³/sec perfiber (0.001 scfm/fiber to about 0.03 scfm/fiber), said aerator meansgenerating bubbles having an average diameter in the range from about0.1 mm to about 25 mm, said bubbles maintain outer surfaces of saidfibers essentially free from build-up of deposits of said particulatematter.
 3. The gas-scrubbed assembly of claim 2 wherein, saidgas-distribution means includes a vertical member centrally axiallydisposed within said skein and through at least one said header; saidlength is from 1% to less than 5% greater than said fixed distance, saidfibers together have a surface area >1 m², each fiber has a length >0.5meter, said fibers together have a surface area in the range from 10 to10³ m², said headers are vertically adjustable to provide said fixeddistance, and, said bubbles are in the size range from 1 mm to 25 mmmeasured in relatively close proximity, in the range from 1 cm to about50 cm, to said through-passages.
 4. The gas-scrubbed assembly of claim 2wherein, each header includes both, a fiber-setting form to hold and setsaid fibers in a chosen pattern, and spacer means to maintain desiredfiber-to-fiber spacing within said skein, said both being integral withsaid header; said fibers are potted within said synthetic resinousmaterial to a depth in the range from about 1 cm to about 5 cm andprotrude through a permeate-discharging face of each said header in arange from about 0.1 mm to about 1 cm.
 5. The gas-scrubbed assembly ofclaim 3 wherein, said permeate collection means includes a verticalmember coaxially disposed within said gas distribution means' verticalmember, said substrate is maintained at a pressure in the range fromabout 1-10 atm, said transmembrane pressure differential is in the rangefrom 3.5 kPa (0.5 psi) to about 175 kPa (25 psi), opposed terminal endportions of said fibers are in open communication with each otherthrough each said header; said fibers are in the range from 0.5 m to 5 mlong, and, said terminal end portions of said fibers are potted withinsaid mass of resin to a depth in the range from about 1 cm to about 5cm.
 6. The gas-scrubbed assembly of claim 5 wherein said particulatematter comprises biologically active microorganisms growing in saidsubstrate.
 7. The gas-scrubbed assembly of claim 5 wherein saidparticulate matter comprises finely divided inorganic particles.
 8. Thegas-scrubbed assembly of claim 1 wherein, each said fiber is formed froma material selected from the group consisting of natural and syntheticpolymers, has an outside diameter in the range from about 20 μm to about3 mm, a wall thickness in the range from about 5 μm to about 2 mm, and,a pore size in the range from 1000 Å to 10000 Å, each said header is acylindrical disc having substantially the same dimensions, and, said gasis a molecular oxygen-containing gas.
 9. In a microfiltration membranedevice, for withdrawing permeate essentially continuously from amulticomponent liquid substrate, said membrane device including: amultiplicity of hollow fiber membranes, or fibers, unconfined in a shellof a module, said fibers being swayable in said substrate, said fibersbeing subject to a transmembrane pressure differential in the range fromabout 0.7 kPa (0.1 psi) to about 345 kPa (50 psi); a first header and asecond header disposed in transversely spaced-apart relationship withsaid second header within said substrate; said first header having aterminal end portion of each fiber secured therein, and said secondheader having an opposed terminal end portion of each fiber securedtherein, all said fibers extending from a permeate-discharging face ofat least one said header; said fibers being sealingly secured with openends of the fibers secured in fluid-tight relationship with each otherin at least one of said headers; permeate collection means to collectsaid permeate through at least one of said headers sealingly connectedin open fluid communication with permeate-discharging faces of saidheaders; and, means for withdrawing said permeate; said fibers, saidheaders and said permeate collection means together forming a verticalcylindrical skein wherein said fibers are essentially verticallydisposed; said first header being upper and disposed in verticallyspaced-apart relationship above said second header, with opposed facesat a fixed distance; each of said fibers having substantially the samelength, said length being from 0.1% to less than 5% greater than saidfixed distance so as to permit restricted displacement of anintermediate portion of each fiber, independently of the movement ofanother fiber; the improvement comprising, each said header having saidfibers spaced apart by a flexible support means having a thicknesscorresponding to a desired lateral spacing between adjacent fibers, saidsupport means extending over only each terminal portion of said fibersnear their ends so as to maintain said ends in closely-spaced apartrelationship.
 10. The membrane device of claim 9 wherein, each saidheader is a mass of synthetic resinous material in which said terminalend portions are potted and said fibers are formed from natural orsynthetic polymers; each said fiber has an outside diameter in the rangefrom about 20 μm to about 3 mm, a wall thickness in the range from about5 μm to about 2 mm, pore size in the range from 1,000 Å to 10,000 Å;and, said displacement is in the lateral or horizontal direction. 11.The membrane device of claim 10 wherein, said permeate collection meansincludes a vertical member axially disposed through said headers andwithin said skein, said substrate is maintained at a pressure in therange from about 1-10 atm, said fibers extend as a skein upwardly from afiber-supporting face of each of said headers, each header hassubstantially the same dimensions, said fibers extend downwardly throughthe permeate-discharging face of said headers, and said permeate isdischarged upwardly relative to said upper header.
 12. The membranedevice of claim 11 wherein, said fibers together have a surface area >1m², each fiber has a length >0.5 meter, said fibers together have asurface area in the range from 10 to 103 m² and, said terminal endportions of said fibers protrude through a permeate-discharging face ofeach said header in a range from about 0.1 mm to about 1 cm.
 13. In aprocess for maintaining the outer surfaces of hollow fiber membranesessentially free from a build-up of deposits of particulate materialwhile separating a permeate from a multicomponent liquid substrate in areservoir, said process comprising, submerging skein fibers in anessentially vertical, cylindrical configuration within said substrate,said fibers being unconfined in a modular shell, and securely held invertically opposed, upper and lower headers spaced-apart at a fixeddistance, said fibers having substantially the same length and from atleast 0.1% greater, to about 5% greater than said fixed distance, atransmembrane pressure differential in the range from about 0.7 kPa (0.1psi) to about 345 kPa (50 psi). and length sufficiently greater than thedirect distance between opposed faces of said first and second headers,so as to present said skein in a swayable configuration above ahorizontal plane through the horizontal center-line of said lowerheader; mounting said headers in fluid-tight open communication withcollection means to collect said permeate; flowing a fiber-cleaning gasthrough a gas-distribution means proximately disposed relative to saidskein, within a zone beneath said skein, and contacting surfaces of saidfibers with sufficient physical impact of bubbles of said gas tomaintain essentially the entire length of each fibers in said skeinawash with bubbles and essentially free from said build-up; maintainingan essentially constant flux through said fibers substantially the sameas an equilibrium flux initially obtained after commencing operation ofsaid process; collecting said permeate in said collection means; and,withdrawing said permeate; the improvement comprising, introducing saidcleansing gas between said fibers within said skein to generate a columnof said bubbles alongside and in contact with outer surfaces of saidfibers; said fibers spaced apart by a flexible support means having athickness corresponding to a desired lateral spacing between adjacentfibers, said support means extending over only each terminal portion ofsaid fibers near their ends, so as to maintain said ends in closelyspaced-apart relationship; restricting movement of said fibers to saidvertical zone defined by lateral movement of outer fibers in said skein;vertically gas-scrubbing said fibers outside surfaces with bubbles whichflow upward in contact with said surfaces; maintaining said surfacessubstantially free from said deposits of particulate matter during aperiod when flux through said fibers has attained equilibrium; andsimultaneously, essentially continuously, withdrawing said permeate. 14.The process of claim 13 wherein, each said hollow fiber has an outsidediameter in the range from about 20 μm to about 3 mm, and a wallthickness in the range from about 5 μm to about 1 mm; said particulatematter is selected from the group consisting of microorganisms andfinely divided inorganic particles; and, said gas-distribution meansdischarges gas in an amount in the range from 0.47-14 cm³/sec per fiber(0.001 scfm/fiber to about 0.03 scfm/fiber) and generates bubbles havingan average diameter in the range from about 1 mm to about 25 mm.
 15. Asystem for withdrawing permeate from a liquid substrate while leavingparticulate matter therein, comprising, (a) a non-pressurized reservoirother than a shell of a module for containing the substrate; (b) anassembly having a plurality of hollow fiber filtering membranes immersedin the substrate each membrane having a length greater than 0.5 m, themembranes together providing a surface area of at least greater than 1 m² and disposed generally vertically between upper and lower generallycylindrical solid bodies comprised of a potting material with (i) thesolid bodies having the membranes sealingly secured therein so as toprevent the substrate from contaminating the permeate, at least aportion of the membranes spaced apart from adjacent membranes by thepotting material to a center to center distance in the range from 1.2 to5 times the outside diameter of the membranes, (ii) lumens of saidmembranes being in fluid communication with a permeate pan connected toone of the solid bodies and immersible in the substrate or to a pair ofpermeate pans connected one to each of the solid bodies and bothimmersible in the substrate, and, (iii) said membranes having a lengthbetween opposed surfaces of the solid bodies, in the range from 0.1 % to5 % greater than the distance between opposed surfaces of the solidbodies; (c) a pump in fluid communication with said lumens of saidmembranes through at least one permeate pan, said pump operable to applya suction to the lumens of the membranes to draw a component of thesubstrate as permeate through said membranes while leaving particulatematter in said substrate; and, (d) a gas-distribution system havingthrough-passages with openings distributed both radially andcircumferentially between the membranes operable to provide a flow a gasthrough the through-passages to produce bubbles in the substrate. 16.The system of claim 15 wherein the length is in the range from 0.1% to 1% greater than the distance between the opposed surfaces of the solidbodies.
 17. The system of claim 16 wherein the gas distribution systemfurther includes a rigid air supply tube for carrying air to thethrough-passages and for spacing and positioning the lower and uppersolid bodies relative to one another.
 18. The system of claim 17 whereinthe air supply tube has additional through-passages along its length.19. The system of claim 15 wherein lower ends of the membranes areplugged.